1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Description of the Related Art
In the prior art, shifting gears in an automatic transmission is typically controlled responsive to detected vehicle speed and throttle opening. In this kind of automatic transmission, however, shifts always follow the same set shift pattern, and the taste of the driver cannot be accommodated. Worse, the shifting pattern cannot be adapted to different road conditions, e.g., a road surface having a changing coefficient of friction or roads over mountainous terrain.
Thus, there has been proposed a control system for an automatic transmission, which provides for modification of a shift pattern in accordance with the taste of the driver or in accordance with the variations in the nature of the road (as disclosed in Japanese Patent Laid-Open No. 261745/1987). In this latter case, the automatic transmission is equipped with a manual shift lever so that the operator can manually execute a shift at a predetermined shift point while the vehicle is running. In response to this manual shift, the control system reads data of the vehicle speed and the throttle opening, compares this data with data for a fundamental shift pattern, and computes a constant in proportion to the difference. This constant is thereafter applied to produce a corresponding change in the fundamental shift pattern. By this manual shifting operation, a new shift pattern can be established in accordance with the taste of the driver or the nature of the road conditions.
Also there has been proposed a control system for an automatic transmission, which corrects the shift pattern by dialogue (voice simulation and voice recognition as disclosed in Japanese Patent Laid-Open No. 134451/1990). In this case, the control system of the automatic transmission corrects the shift pattern by conducting a dialogue with the driver and interpreting the answers of the driver by fuzzy logic. Specifically, the control system of the automatic transmission corrects the shift pattern by fuzzy inference of the driver's answers, by computing a correction coefficient from the inferred answers, and by multiplying the vehicle speed by the correction coefficient so that the shift point is determined by the product of the vehicle speed and the correction coefficient.
In the prior art automatic transmission control system in which the shift pattern is modified by a manual shifting operation, the fundamental shift pattern is changed by a single constant of proportion. Thus, one shift operation from a first selected gear to a second selected gear can be set according to the driver's taste or the road conditions, but shift operations other than from the first selected gear to the second selected gear are not necessarily set according to the driver's taste or the road conditions. Further in case the shifting pattern is to be changed in accordance with the road conditions, it is difficult by manual shifting to properly set the new shift pattern for the actual road conditions. Specifically, the driver will not always manually shift at the same shift point because the shift point varies to a certain extent. Moreover, the driver may shift at an improper time so that the shift pattern is changed to the data of the vehicle speed and the throttle opening at an improper shift point for the present road conditions.
Similarly in the prior art shift control which is set by dialogue, a single correction coefficient is computed from the dialogue answers by the fuzzy inference so that the shift pattern is corrected to correspond to this single correction coefficient. As a result, what is obtained is only one shift pattern but not a variety of shift patterns according to the driver's taste and the road situations.
Also, the correction coefficient derived by fuzzy inference is based on only the variables of vehicle speed and throttle opening so that the shift pattern intended by the driver is difficult to achieve. If the number of variables would be increased, more questions would have to be answered by the driver in the dialogue. As the number of questions increases, the driver requires more special knowledge of the vehicle and the road conditions so that the driver finds it increasingly difficult to properly answer the questions.